For solid state and hybrid breakers, the current changes path one or more times during the operation of the breaker. This change of current path is referred to as current commutation. In the solid state breaker, the current normally flows in a path with an IGBT (Insulated Gate Bipolar Transistor) or other similar component. Upon IGBT turn-off, the current cannot be interrupted instantly due to the inductance of the circuit. The current is thus commutated to a path parallel with the IGBT. The parallel path can contain a snubber circuit including e.g. a Metal Oxide Varistor, MOV. When the current has reached zero in the IGBT, the current commutation is completed.
Similar commutations are made in hybrid breakers, where the current flows in a mechanical switch branch during normal closed operation. When the mechanical switch opens, the current is first commutated to a parallel solid state breaker having an IGBT and another parallel path with a snubber. A hybrid breaker of this type will thus have two current commutations; first the current commutates from the mechanical switch to the IGBT, and then from the IGBT to the snubber.
WO 2011/044928 discloses a hybrid circuit breaker known in the art. The circuit breaker comprises a main current path with a mechanical switch element, a commutation path parallel to the main path and comprising a controllable semiconductor switch element, as well as a dissipative circuit arranged in parallel with the main path and the commutation path and provided with a suitable dissipative element, such as a varistor or the like.
When faults occur, the current through the mechanical switch element increases, making it increasingly difficult to commutate the current from the mechanical switch element. Hence, in the prior art it is increasingly difficult to open the mechanical switch element, before the current through the mechanical switch element has grown too large. For hybrid breakers, this requires a fast current commutation away from the mechanical breaker path.